BiVO₄–Liquid Junction Photovoltaic Cell with 0.2% Solar Energy Conversion Efficiency

Abstract

BiVO4 is an important photoanode material for water oxidation, but its photoelectrochemistry regarding the triiodide/iodide redox couple is not well understood. Here, we use a combination of open circuit potential measurements, photoelectrochemical scans, and liquid surface photovoltage spectroscopy (SPS) to confirm that BiVO4/triiodide/iodide electrolyte contacts produce up to 0.55 V photovoltage under 23 mW/cm–2 illumination from a 470 nm LED. Inspired by these results, we construct FTO/BiVO4/KI­(I2)aq/Pt sandwich photoelectrochemical cells from electrochemically grown 0.5 × 0.5 cm2 BiVO4 and Mo-doped BiVO4 films. Under AM 1.5 illumination, the devices have up to 0.22% energy conversion efficiency, 0.32 V photovoltage, and 1.8 mA cm–2 photocurrent. Based on SPS, hole transfer to iodide is sufficiently fast to prevent the competing water oxidation reaction. Mo doping increases the incident photon-to-current efficiency to up to 55% (at 425 nm under front illumination) by improving the BiVO4 conductivity, but this comes at the expense of a lower photovoltage resulting from recombination at the Mo defects and a detrimental Schottky junction at the interface with FTO. Additional photovoltage losses are caused by the offset between the BiVO4 valence band edge and the triiodide/iodide electrochemical potential and by electron back transfer to iodide at the FTO back contact (shunting). Overall, this work provides the first example of a BiVO4–liquid photovoltaic cell and an analysis of its limitations. Even though the larger band gaps of metal oxides constrain their solar energy conversion efficiency, their transparency to visible light and deep valence bands makes them suitable for tandem photovoltaic devices.

14.8% Quantum Efficient Gallium Phosphide Photocatalyst for Hydrogen Evolution

“…The SPV signal represents the change of the CPD, SPV = CPD (light) – CPD (dark), resulting from light-induced charge separation. It provides an estimate of the photovoltage of the sample, that is the difference of the electrochemical potentials of the majority and minority carriers at the sample surfaces. − SPV spectra are shown in Figure a together with the logarithmic plot of the xenon arc lamp light intensity. For all samples, negative SPV signals are observed (electrons move away from the Kelvin probe), in agreement with charge separation within the electron depletion layer at the n-GaP surface, as shown in Figure b.…”

Effect of ZnO and PEDOT:PSS charge selective layers on photovoltage of cuprous oxide (Cu2O) heterojunction solar cells

“…the open circuit potential of the corresponding solar cell. 43–46 SPV spectra for ZnO/CuO 2 films on FTO and AZO are shown in Fig. 8a and b.…”

Electrochemical versus Photoelectrochemical Water Oxidation Kinetics on Bismuth Vanadate (Photo)anodes

“…The PEC reaction mechanism can be understood thermodynamically by light irradiation, resulting in a splitting of the electron and hole quasi-Fermi levels ( E F,n and E F,p , respectively) by the photovoltage V photo, . The simplest assays of V photo derive from comparison of light and dark current–voltage ( J – V ) curves and onset potentials, while surface photovoltage measurements provide an alternative assay. − However, such measurements do not provide direct assays of the underlying Fermi level splitting, complicating their interpretation. In the study herein, we address these uncertainties as well as addressing whether a population density model based on a chemical rate-determining step is applicable to both EC and PEC water oxidation on BiVO 4.…”

“…It is apparent that these two plots of the current density versus E F,p overlap, strongly supporting the validity of our determination of V photo and emphasizing that the water oxidation current is determined only by the position of E F,p , and therefore p s , independent of whether the hole population is generated electrochemically or photoelectrochemically. In contrast to other approaches to determine the photovoltage, − the experimental approach demonstrated herein provides an operando determination of hole quasi-Fermi level as a function of light intensity, opening up the potential for in depth analyses of photovoltage generation.…”